JP7846958B2 - Electrode forming materials, electrode mixtures, electrodes for energy devices, and energy devices - Google Patents

Description

Applicable under Article 30, Paragraph 2 of the Patent Act. Published on the Hitachi Chemical Co., Ltd. website on February 3, 2020. Published on the "Ipros Monozukuri" database site operated by Ipros Co., Ltd. on February 13, 2020.

Embodiments of the present invention relate to electrode forming materials, electrode mixtures, electrodes for energy devices, and energy devices.

Lithium-ion rechargeable batteries are widely used as power sources for portable information terminals such as notebook computers, mobile phones, and PDAs (Personal Digital Assistants), as well as for electric vehicles. Lithium-ion rechargeable batteries are non-aqueous electrolyte-based energy devices with high energy density.

The electrodes for lithium-ion secondary batteries are manufactured, for example, as follows: First, an electrode slurry is prepared by kneading the active material, binder, and solvent. This electrode slurry is then applied to one or both sides of a metal foil current collector using a transfer roll or the like, and the solvent is removed to form an electrode slurry layer. Subsequently, the electrode slurry layer is compressed and molded using a roll press or the like to produce the electrode.

Patent Document 1 describes a binder resin material for energy device electrodes containing a copolymer comprising structural units derived from (meth)acrylonitrile and structural units derived from a compound having two or more ethylenically unsaturated bonds. Patent Document 2 describes a copolymer for energy device electrodes containing a copolymer comprising structural units derived from (meth)acrylonitrile and structural units derived from (meth)acrylic acid ester, wherein the copolymer has a swelling degree of 200 to 400% in relation to the electrolyte.

International Publication No. 2014/098233 Japanese Patent Publication No. 2016-143635

The binder used in the manufacture of electrodes is required to have excellent properties such as adhesion between active materials and between active materials and current collectors, as well as electrochemical stability. Furthermore, to increase the capacity of lithium-ion secondary batteries, it is necessary to satisfy these properties with smaller amounts of binder additive.

On the other hand, in recent years, with the increasing capacity of lithium-ion secondary batteries, the use of silicon-based active materials as negative electrode active materials is being considered. Using silicon-based active materials as negative electrode active materials is expected to significantly improve the discharge capacity of lithium-ion secondary batteries.

Under these circumstances, embodiments of the present invention aim to provide an electrode-forming material and an electrode mixture capable of manufacturing electrodes for energy devices with excellent cycle characteristics. Furthermore, embodiments of the present invention aim to provide electrodes capable of manufacturing energy devices with excellent cycle characteristics. Moreover, embodiments of the present invention aim to provide energy devices with excellent cycle characteristics.

Examples of embodiments of the present invention are listed below. The present invention is not limited to the following embodiments.
(1) An electrode forming material that contains polyamide-imide and is used to form an electrode containing an active material comprising a silicon-based active material and a carbon-based active material.
(2) The electrode-forming material according to (1) above, wherein the number-average molecular weight of the polyamide-imide is 3,000 to 100,000.
(3) The electrode forming material according to (1) or (2) above, wherein the content of the silicon-based active material is 10 to 50% by mass, based on the mass of the active material.

(4) An electrode mixture containing the electrode forming material described in any one of the above items (1) to (3), and an active material including a silicon-based active material and a carbon-based active material.
(5) The electrode mixture according to (4) above, wherein the content of the silicon-based active material is 10 to 50% by mass, based on the mass of the active material.

(6) An electrode for an energy device, comprising a current collector and an electrode mixture layer formed on at least a portion of the surface of the current collector using the electrode mixture described in (4) or (5) above.
(7) An energy device having a positive electrode and a negative electrode, wherein at least one of the positive electrode and the negative electrode includes an electrode for an energy device as described in (6) above.

According to embodiments of the present invention, it is possible to provide an electrode-forming material and an electrode mixture capable of manufacturing electrodes for energy devices with excellent cycle characteristics. Furthermore, according to embodiments of the present invention, it is possible to provide electrodes capable of manufacturing energy devices with excellent cycle characteristics. Moreover, according to embodiments of the present invention, it is possible to provide energy devices with excellent cycle characteristics.

Embodiments of the present invention will now be described. The present invention is not limited to the following embodiments.

<Material for electrode formation>
An electrode-forming material according to an embodiment of the present invention is used to form an electrode containing an active material comprising a silicon-based active material and a carbon-based active material. The electrode-forming material contains at least polyamide-imide.

Silicon-based active materials include silicon-containing alloys and silicon oxides. While lithium-ion secondary batteries using silicon-based active materials offer the potential for significantly improved discharge capacity, they tend to have difficulty achieving sufficiently good cycle characteristics. One possible reason is that the large volume expansion and contraction of silicon-based active materials during charging and discharging can prevent sufficient adhesion between active materials or between active materials and current collectors, leading to detachment of the active material from the electrodes, electrode deformation, or cracking. Furthermore, another possible cause is that cracking or peeling of the protective film (hereinafter sometimes referred to as "SEI" (Solid Electrolyte Interphase)) formed on the surface of the active material during charging and discharging can occur, forming an active surface and leading to the decomposition of the electrolyte.

As a solution to the above, it is conceivable to use a binder that ensures sufficient adhesion between active materials or between active materials and current collectors even when volume expansion and contraction occurs due to charging and discharging, and that can sufficiently suppress cracking of the SEI (Single Interconnected Insulator). In the embodiment of the present invention, the electrode forming material contains polyamide-imide as such a binder.

Polyamide-imides are polymers containing both amide and imide bonds within their molecules. Polyamide-imides are obtained by reacting at least a tricarboxylic acid anhydride or its derivative (hereinafter sometimes referred to as the "acid component") with a diisocyanate compound and/or a diamine compound. Other arbitrary compounds may also be reacted. The reaction may use only one of each starting compound, or a combination of two or more.

The acid component is not particularly limited, including derivatives thereof, as long as it is a trivalent carboxylic acid having an acid anhydride group that reacts with an isocyanate group or an amino group. Considering heat resistance, compounds having an aromatic ring group are preferred. Examples of tricarboxylic acid anhydrides include compounds represented by the following formulas (I) or (II). Considering heat resistance, cost, etc., it is particularly preferable that the acid component includes trimellitic anhydride. The acid components are used individually or in mixtures depending on the purpose.

Y represents _-CH₂- , -CO-, _-SO₂- , or -O-.

The acidic component may include tetracarboxylic anhydrides as needed. Examples of tetracarboxylic anhydrides include tetracarboxylic dianhydrides (pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 2,3,5,6-pyridinetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, 4,4'-sulfonyl diphthalic dianhydride, m-terphenyl-3,3',4,4'-tetracarboxylic dianhydride, 4,4'-oxydiphthalic dianhydride, 1,1,1,3,3,3-hexafluoro-2,2 Examples include -bis(2,3- or 3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(2,3- or 3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis[4-(2,3- or 3,4-dicarboxyphenoxy)phenyl]propane dianhydride, 1,1,1,3,3,3-hexafluoro-2,2-bis[4-(2,3- or 3,4-dicarboxyphenoxy)phenyl]propane dianhydride, 1,3-bis(3,4-dicarboxyphenyl)-1,1,3,3-tetramethyldisiloxane dianhydride, butanetetracarboxylic acid dianhydride, bicyclo-[2,2,2]-octo-7-ene-2:3:5:6-tetracarboxylic acid dianhydride, etc.

Examples of diisocyanate compounds and/or diamine compounds include those represented by the following formulas (III), (IV), or (V). Diisocyanate compounds and/or diamine compounds are used individually or in combination, depending on the purpose.

In these formulas, ^R2 is a hydrogen atom, an alkyl group, a hydroxyl group, or an alkoxy group, and ^R3 is an isocyanate group or an amino group. Here, the alkyl group and alkoxy group that is ^R2 preferably have 1 to 20 carbon atoms.

Examples of compounds represented by formulas (III), (IV), or (V) include 4,4'-diisocyanatodiphenylmethane, 4,4'-diisocyanatobiphenyl, 3,3'-diisocyanatobiphenyl, 3,4'-diisocyanatobiphenyl, 4,4'-diisocyanato-3,3'-dimethylbiphenyl, 4,4'-diisocyanato-2,2'-dimethylbiphenyl, 4,4'-diisocyanato-3,3'-diethylbiphenyl, 4,4'-diisocyanato-2,2'-diethylbiphenyl, 4,4'-diisocyanato-3,3'-dimethoxybiphenyl, 4,4'-diisocyanato-2,2'-dimethoxybiphenyl, 1,5 Examples include -diisocyanatonaphthalene, 2,6-diisocyanatonaphthalene, 4,4'-diaminodiphenylmethane, 4,4'-diaminobiphenyl, 3,3'-diaminobiphenyl, 3,4'-diaminobiphenyl, 4,4'-diamino-3,3'-dimethylbiphenyl, 4,4'-diamino-2,2'-dimethylbiphenyl, 4,4'-diamino-3,3'-diethylbiphenyl, 4,4'-diamino-2,2'-diethylbiphenyl, 4,4'-diamino-3,3'-dimethoxybiphenyl, 4,4'-diamino-2,2'-dimethoxybiphenyl, 1,5-diaminonaphthalene, 2,6-diaminonaphthalene, etc.

Other aromatic diisocyanate compounds or aromatic diamine compounds other than those represented by formulas (III), (IV), or (V) can be used as the diisocyanate compound and/or diamine compound. Examples of other aromatic diisocyanate compounds or aromatic diamine compounds include tolylene diisocyanate, xylylene diisocyanate, 4,4'-diisocyanatodiphenyl ether, 2,2-bis[4-(4'-isocyanatophenoxy)phenyl]propane, tolylenediamine, xylylenediamine, 4,4'-diaminodiphenyl ether, and 2,2-bis[4-(4'-aminophenoxy)phenyl]propane.

As the diisocyanate compound and/or diamine compound, aliphatic or alicyclic diisocyanate compounds and/or aliphatic or alicyclic diamine compounds can be used. Examples of these compounds include hexamethylenediamine, 2,2,4-trimethylhexamethylenediamine, diaminoisophorone, bis(4-aminocyclohexyl)methane, 1,4-diaminotranscyclohexane, hydrogenated m-xylylenediamine, hexamethylenediisocyanate, 2,2,4-trimethylhexamethylenediisocyanate, diisocyanatoisophorone, bis(4-isocyanatocyclohexyl)methane, 1,4-diisocyanatotranscyclohexane, and hydrogenated m-xylylenediisocyanate. When using aliphatic or alicyclic diisocyanate compounds and/or aliphatic or alicyclic diamine compounds, it is preferable to use aromatic diisocyanate compounds and/or aromatic diamine compounds in combination. From the viewpoint of the heat resistance of the resulting polyamide-imide, the content of these compounds in the total amount of diisocyanate compounds and/or diamine compounds is preferably 50 mol% or less.

Considering the balance of heat resistance, solubility, mechanical properties, and cost, the diisocyanate compound and/or diamine compound is particularly preferably 4,4'-diphenylmethane diisocyanate. In addition to the diisocyanate compound and/or diamine compound, isocyanate compounds and/or amine compounds with three or more functionalities can also be used in combination.

To avoid changes over time, if necessary, compounds with stabilized isocyanate groups using a blocking agent may be used. Examples of blocking agents include alcohols, phenols, and oximes, but there are no particular restrictions.

The acid component (hereinafter sometimes referred to as component (a)) and the diisocyanate compound and/or diamine compound (hereinafter sometimes referred to as component (b)) are preferably reacted such that, when carboxyl groups, acid anhydride groups, and reactive hydroxyl groups are present, the ratio of the total number of isocyanate groups and amino groups to the total number of these functional groups is preferably 0.6 to 1.4, more preferably 0.7 to 1.3, and even more preferably 0.8 to 1.2. A ratio of 0.6 or higher tends to facilitate increasing the molecular weight of the polyamide-imide. A ratio of 1.4 or lower tends to prevent vigorous foaming reactions and the resulting large amount of unreacted material, thus making it easier to obtain good stability for the polyamide-imide.

The amount of solvent used during the reaction is preferably 100 to 300 parts by mass, and more preferably 150 to 250 parts by mass, per 100 parts by mass of the total amount of components (a) and (b). Using 100 parts by mass or more of solvent tends to prevent reactions accompanied by foaming. Using 300 parts by mass or less of solvent tends to prevent the synthesis time from becoming too long, and also tends to result in a sufficient concentration of polyamide-imide in the solution obtained after synthesis. Suitable solvents include, for example, polar solvents such as N-methyl-2-pyrrolidone, N,N'-dimethylformamide, γ-butyrolactone, N,N'-dimethylpropylene urea [1,3-dimethyl-3,4,5,6-tetrahydropyridimine-2(1H)-one], dimethyl sulfoxide, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and sulfolane; aromatic hydrocarbon solvents such as xylene and toluene; and ketone solvents such as methyl ethyl ketone and methyl isobutyl ketone.

The reaction between component (a) and component (b) may or may not require the use of a catalyst, either from the viewpoint of reaction efficiency or from the viewpoint of reducing residual impurities. For example, if a fluorine-based catalyst such as potassium fluoride is not used, contamination by fluorine is prevented, resulting in a polyamide-imide with excellent properties such as electrochemical stability.

The synthesis conditions for polyamide-imides are diverse and cannot be generalized, but for example, they can be carried out at temperatures of 80 to 180°C. Synthesis is preferably carried out under a nitrogen atmosphere to reduce the influence of moisture in the air.

The synthesized polyamide-imide can be obtained, for example, as a polyamide-imide solution dissolved in the solvent used in the reaction.

The polyamide-imide preferably has a number-average molecular weight of 3,000 to 100,000. A number-average molecular weight of 3,000 or higher tends to improve various properties such as viscosity and strength. The number-average molecular weight of the polyamide-imide may be 5,000 or higher, 8,000 or higher, 15,000 or higher, or 20,000 or higher. A number-average molecular weight of 100,000 or lower tends to easily provide good adhesion to the current collector. The number-average molecular weight of the polyamide-imide may be 80,000 or lower, 50,000 or lower, 30,000, or 27,000 or lower.

The polyamide-imide preferably has a weight-average molecular weight of 3,000 to 300,000. A weight-average molecular weight of 3,000 or higher tends to improve various properties such as viscosity and strength. The weight-average molecular weight of the polyamide-imide may be 5,000 or higher, 8,000 or higher, or 20,000 or higher. A weight-average molecular weight of 300,000 or lower tends to easily provide good adhesion to the current collector. The weight-average molecular weight of the polyamide-imide may be 240,000 or lower, 150,000 or lower, 90,000 or lower, or 80,000 or lower.

The polyamide-imide preferably has a dispersion degree (Mw/Mn) of 2.8 or less. A lower dispersion degree tends to result in the formation of electrodes with superior properties. The lower limit of the dispersion degree may be 1.0 or higher. A dispersion degree of 1.1 to 2.5 is more preferable, and 1.1 to 2.3 is even more preferable.

The number-average and weight-average molecular weights of polyamideimides can be measured using gel permeation chromatography (GPC) with a calibration curve for standard polystyrene. These values can be controlled and adjusted by repeatedly sampling and measuring during synthesis, continuing the synthesis until the target number-average molecular weight is reached.

The polyamide-imide may have a tensile modulus of 1.8 GPa or higher, 2.0 GPa or higher, 2.1 GPa or higher, 2.5 GPa or higher, 2.7 GPa or higher, or 3.0 GPa or higher. Using a polyamide-imide with a tensile modulus of 1.8 GPa or higher can further improve adhesion between active materials and between active materials and current collectors, tending to improve reliability. The upper limit of the tensile modulus is not particularly limited, but for example, it may be 3.5 GPa or lower, 3.4 GPa or lower, 3.3 GPa or lower, or 3.0 GPa or lower. The tensile modulus of the polyamide-imide is measured using a 20 μm thick polyamide-imide film formed using the polyamide-imide. Specifically, it can be measured at room temperature using an autograph according to the method described in the examples.

The polyamide-imide may have a tensile strength of 80 MPa or higher, 90 MPa or higher, 100 MPa or higher, 110 MPa or higher, or 120 MPa or higher. When the tensile strength is 80 MPa or higher, the adhesion between active materials and between active materials and current collectors can be further improved, and reliability tends to be further enhanced. The upper limit of the tensile strength is not particularly limited, but for example, it may be 150 MPa or lower, 140 MPa or lower, or 130 MPa or lower. The tensile strength of the polyamide-imide is measured using a 20 μm thick polyamide-imide film formed using the polyamide-imide. Specifically, it can be measured using an autograph according to the method described in the examples.

The electrode forming material contains at least polyamide-imide and may further contain known electrode binders such as polyolefins and acrylic polymers. Polyamide-imide is less susceptible to volume changes in silicon-based active materials, thus preventing electrode deformation, and exhibits excellent adhesion, improving electrode reliability. Furthermore, polyamide-imide has good chemical resistance, making it less susceptible to the effects of electrolytes.

<Electrode mixture>
An electrode mixture according to an embodiment of the present invention contains at least an active material comprising a silicon-based active material and a carbon-based active material, and the electrode forming material of the above embodiment. The electrode mixture may be a slurry containing a solvent. The electrode mixture may contain any components such as conductive materials and additives.

The active material comprises at least a silicon-based active material and a carbon-based active material. The silicon-based active material contains at least silicon and includes, for example, silicon-containing alloys, silicon-containing oxides, silicon-containing nitrides, or silicon-containing carbides. Examples of silicon-containing alloys include alloys containing silicon and at least one selected from the group consisting of tin, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony, and chromium. Specific examples of silicon-containing oxides include SiO, _SiO₂ , _LiSiO , etc., specific examples of silicon-containing nitrides include _Si₃N₄ , _Si₂N₂O , etc., and examples of silicon-containing carbides include SiC _, etc.

The carbon-based active material may be a carbon material, such as amorphous carbon material, natural graphite, composite carbon material obtained by forming an amorphous carbon material coating on natural graphite, or artificial graphite (graphite obtained by calcining resin raw materials such as epoxy resin or phenolic resin, or pitch-based raw materials obtained from petroleum, coal, etc.). It is preferable that the carbon-based active material contains natural graphite and/or artificial graphite.

The silicon-based active material content is preferably 10% by mass or more, based on the mass of the active material. When the silicon-based active material content is 10% by mass or more, there is a tendency to be able to increase the discharge capacity. The silicon-based active material content may be 15% by mass or more, 20% by mass or more, 25% by mass or more, or 30% by mass or more. The silicon-based active material content may be less than 100% by mass, based on the mass of the active material. Including a carbon-based active material in the active material can prevent cycle-specific degradation and tends to improve handling. The silicon-based active material content may be 80% by mass or less, 70% by mass or less, 50% by mass or less, 45% by mass or less, 40% by mass or less, 35% by mass or less, or 30% by mass or less.

Examples of solvents include those exemplified as solvents usable in the synthesis of polyamide-imides. The solvent used in the synthesis of polyamide-imides may be the same as the solvent contained in the electrode mixture. The active material content is preferably 80% by mass or more, based on the total mass of components other than the solvent contained in the electrode mixture. When the active material content is 80% by mass or more, the discharge capacity tends to improve further. The active material content may be 85% by mass or more, 88% by mass or more, 90% by mass or more, 93% by mass or more, or 95% by mass or more. Considering the binder content, the active material content may be, for example, 99% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, or 95% by mass or less, based on the total mass of components other than the solvent contained in the electrode mixture.

Examples of conductive materials include carbon black such as acetylene black and Ketjenblack, graphite, graphene, and carbon nanotubes. As the active material, materials capable of functioning as an active material, such as materials with a structure capable of intercalating and releasing electrolytes such as lithium ions, can be selected and used. Conversely, as the conductive material, materials that cannot function as an active material, such as materials without a structure capable of intercalating and releasing electrolytes such as lithium ions, can be selected and used. When the electrode mixture contains a conductive material, the content of the conductive material may be, for example, 0.01% by mass or more, 0.1% by mass or more, or 1% by mass or more, based on the total mass of components other than the solvent contained in the electrode mixture. The content of the conductive material may also be, for example, 50% by mass or less, 30% by mass or less, or 15% by mass or less, based on the total mass of components other than the solvent contained in the electrode mixture.

<Electrodes for energy devices>
An electrode for an energy device according to an embodiment of the present invention comprises at least a current collector and an electrode mixture layer formed on at least a portion of the surface of the current collector. The electrode mixture layer can be formed using the electrode mixture of the above embodiment.

Examples of current collector materials include copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymers, conductive glass, and aluminum-cadmium alloys. The surface of the current collector may be treated with carbon, nickel, titanium, silver, etc., to improve adhesion, conductivity, and reduction resistance. Examples of current collector shapes include plates and films.

When the current collector is in the form of a plate or film, the electrode comprises at least the current collector and an electrode mixture layer formed on one or both surfaces of the current collector. The electrode mixture layer can be formed, for example, by the following method: First, an electrode mixture is prepared by mixing at least an active material containing a silicon-based active material and a carbon-based active material, an electrode-forming material, and a solvent. Next, the electrode mixture is applied to at least one surface of the current collector to form a coating film. Then, the solvent contained in the coating film is evaporated. After the solvent has evaporated, the coating film is compression-molded. Examples of application methods include the doctor blade method, dipping method, spray method, and transfer roll method. An example of a compression molding method is the roll press method.

Compression molding can be performed while heating the coating film. Compression molding may be performed at room temperature or under heating, and then the coating film may be heated afterward. The heating temperature may be, for example, 200°C or higher, 250°C or higher, or 260°C or higher. The heating temperature may also be 300°C or lower, 290°C or lower, or 280°C or lower. By using the electrode-forming material of the above embodiment, an electrode mixture layer with good adhesion between active materials and between active materials and current collectors can be formed even at low heating temperatures. For example, the heating temperature can be reduced by approximately 30 to 100°C compared to when polyimide is used as the binder.

<Energy Devices>
An energy device according to an embodiment of the present invention has a positive electrode and a negative electrode, and at least one of the positive electrode and the negative electrode includes an electrode for the energy device according to the embodiment. Examples of energy devices include non-aqueous electrolyte secondary batteries and capacitors. The non-aqueous electrolyte secondary battery is preferably a lithium-ion secondary battery.

A non-aqueous electrolyte secondary battery comprises, for example, an electrode group including a positive electrode, a negative electrode, and a separator, and a battery casing that houses the electrode group. The battery casing is filled with an electrolyte. A non-aqueous electrolyte secondary battery may be a so-called laminated type battery, or a battery of a different shape (coin type, cylindrical type, stacked type, etc.).

Examples of separators include, but are not limited to, polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and nonwoven fabrics treated with a hydrophilic coating. The battery casing may be, for example, a container formed from a laminate film. Examples of laminate films include a laminated film in which a resin film such as polyethylene terephthalate (PET film), a metal foil such as aluminum, copper, or stainless steel, and a sealant layer such as polypropylene are laminated in that order.

The electrolyte contains, for example, an electrolyte salt and a non-aqueous solvent. The electrolyte salt may be a lithium salt. Examples of lithium salts include at least _one selected from _the group _consisting of _LiPF₂₆ , _LiBF₄ , _LiClO₄ , LiB( _{C₆H₅ ) ₄} , _LiCH₃SO₃ , _{CF₃SO₂OLi} , LiN( _SO₂F ) _₂ (Li[FSI], lithium bisfluorosulfonyliimide), LiN( _SO₂CF₃ ) _₂ (Li[TFSI], lithium _{bistrifluoromethanesulfonyliimide} ), and LiN( _{SO₂CF₂CF₃} ) _₂ . The non-aqueous solvent may be, for example, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, acetonitrile, 1,2-dimethoxyethane, dimethoxymethane, tetrahydrofuran, dioxolane, methylene chloride, methyl acetate, etc. The electrolyte may contain additives such as vinylene carbonate.

The energy device includes the energy device electrode of the embodiment as at least one of the positive and negative electrodes. Preferably, at least the negative electrode is the energy device electrode of the embodiment. If the energy device includes an electrode other than the energy device electrode of the embodiment, the electrode may be a general electrode used in the field of energy devices. For example, a general positive electrode has at least a current collector and a positive electrode mixture layer formed on at least a portion of the surface of the current collector. The positive electrode mixture layer contains at least a positive electrode active material and a binder, and may further contain conductive materials, etc.

The positive electrode active material may be, for example, nickel cobalt manganese oxide (NCM), lithium cobalt dioxide (LCO), nickel cobalt aluminum oxide (NCA), lithium manganese oxide (LMO), etc. Examples of the current collector and conductive material include the current collector and conductive material exemplified in the electrode for the energy device of the above embodiment.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

<Example 1>
(Preparation of electrode-forming material (polyamide-imide resin))
120.0 g of trimellitic anhydride, 158.0 g of 4,4'-diphenylmethane diisocyanate, and 525.3 g of N-methyl-2-pyrrolidone (NMP) were placed in a flask equipped with a thermometer, stirrer, and condenser, and the temperature was gradually raised to 130°C over 2 hours while stirring in a dry nitrogen stream. While paying attention to the rapid effervescence of carbon dioxide gas produced by the reaction, the temperature was maintained at 130°C and heating was continued for 6 hours, after which the reaction was stopped to obtain a polyamide-imide solution.

The non-volatile content (at 200°C - 2h) of the obtained polyamide-imide solution was 31.8% by mass, and the viscosity (at 25°C) was 5.0 Pa·s. The number-average molecular weight of the polyamide-imide was 18,000. The number-average molecular weight was measured by gel permeation chromatography (GPC) using a calibration curve for standard polystyrene under the following conditions.

Model: Hitachi, Ltd. Product Name: L6000
Detector: Hitachi, Ltd. Product name: L4000 UV
Wavelength: 270nm
Data processing machine: ATT 8
Columns: Manufactured by Hitachi Chemical Co., Ltd. Product name: Gelpack GL-S300MDT-5 x 2
Column size: 8 mm diameter x 300 mm
Solvent: DMF/THF = 1/1 (liter) + 0.06 M phosphoric acid + 0.06 M lithium bromide. Sample concentration: 5 mg/1 ml
Injection volume: 5μl
Pressure: 4.8 × 10⁶ Pa (49 kgf/ ^cm² )
Flow rate: 1.0ml/min

The obtained polyamide-imide solution was applied to a glass substrate to form a coating. After drying the coating by heating at 80°C for 30 minutes, a polyamide-imide film with a thickness of 20 μm was prepared by heating at 270°C for 30 minutes. The polyamide-imide film was peeled from the glass substrate, and strip-shaped test pieces measuring 60 mm in length and 10 mm in width were cut out. Tensile tests were performed on the obtained test pieces at room temperature using an Autograph (AGS-5kNG, Shimadzu Corporation) to determine the mechanical properties (tensile strength and modulus of elasticity). The tensile tests were performed with a chuck distance of 20 mm and a tensile speed of 5 m/min. The tensile strength was 116 MPa, and the tensile modulus of elasticity was 2.1 GPa.

(Fabrication of the negative electrode)
A silicon-based active material (Si alloy) and a carbon-based active material (graphite, "SMGYM2" manufactured by Hitachi Chemical Co., Ltd.) were used as the negative electrode active material. The negative electrode active material and a solution (polyamide-imide solution) containing the electrode forming material and NMP obtained above were mixed so that the solid content ratio (carbon-based active material: silicon-based active material: polyamide-imide) was 66.5 mass%: 28.5 mass%: 5.00 mass% so that the capacity density was 600 mAh/kg. NMP was further added to adjust the viscosity to obtain a slurry-like negative electrode mixture. The obtained negative electrode mixture was applied substantially evenly and homogeneously to one side of a current collector (metal foil (Cu, 10 μm)). After that, the coating film was dried, compressed and molded by pressing, and then cured by heating at 270°C for 30 minutes to obtain the negative electrode.

(Battery fabrication and evaluation)
A positive electrode was obtained having a positive electrode mixture layer containing NCM111 (nickel cobalt manganese oxide, manufactured by BASF Toda Battery Materials LLC) as the positive electrode active material, and a current collector (metal foil (Al, 15 μm)). Using this positive electrode and the negative electrode obtained above, a battery containing an electrolyte (ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (1 vol/2 vol) + vinylene carbonate (VC) (1 wt%)) was fabricated. When the charge-discharge cycle characteristics of the battery were measured, the capacity retention rate after 100 cycles was 83.2%.

<Example 2>
(Preparation of electrode-forming material (polyamide-imide resin))
134.5 g of trimellitic anhydride, 176.9 g of 4,4'-diphenylmethane diisocyanate, and 586.9 g of N-methyl-2-pyrrolidone were placed in a flask equipped with a thermometer, stirrer, and condenser, and the temperature was gradually raised to 130°C over 2 hours while stirring in a dry nitrogen stream. While paying attention to the rapid effervescence of carbon dioxide gas produced by the reaction, the temperature was maintained at 130°C and heating was continued for 6.5 hours, after which the reaction was stopped to obtain a polyamide-imide solution.

The non-volatile content (at 200°C - 2h) of the obtained polyamide-imide solution was 33.1% by mass, and its viscosity (at 25°C) was 15.0 Pa·s. The number-average molecular weight of the polyamide-imide was 24,000.

(Fabrication and evaluation of lithium-ion secondary batteries)
A battery was fabricated in the same manner as in Example 1, and the charge-discharge cycle characteristics of the battery were measured. The capacity retention rate after 100 cycles was 73.0%.

<Example 3>
(Preparation of electrode-forming material (polyamide-imide resin))
88.4 g of trimellitic anhydride, 46.5 g of 4,4'-diphenylmethane diisocyanate, 73.7 g of 3,3'-dimethoxybiphenyl-4,4'-diisocyanate, and 679.3 g of N-methyl-2-pyrrolidone were placed in a flask equipped with a thermometer, stirrer, and condenser, and the mixture was gradually heated to 130°C over 2 hours while stirring in a dry nitrogen stream. While paying attention to the rapid effervescence of carbon dioxide produced by the reaction, the mixture was maintained at 130°C and heating continued for 6 hours, after which the reaction was stopped to obtain a polyamide-imide solution.

The non-volatile content (at 200°C - 2h) of the obtained polyamide-imide solution was 21.2% by mass, and its viscosity (at 25°C) was 7.4 Pa·s. The number-average molecular weight of the polyamide-imide was 28,000.

(Battery fabrication and evaluation)
A battery was fabricated in the same manner as in Example 1, and the charge-discharge cycle characteristics of the battery were measured. The capacity retention rate after 100 cycles was 84.4%.

<Comparative Example 1>
A battery was fabricated using a commercially available water-soluble resin in the same manner as in Example 1. When the charge-discharge cycle characteristics of the battery were checked, the capacity retention rate after 100 cycles was 1.6%.

<Comparative Example 2>
A battery was fabricated in the same manner as in Example 1, except that a polyimide solution (polyamic acid type polyimide resin, "HCI-7000" manufactured by Hitachi Chemical Co., Ltd.) was used. When the charge-discharge cycle characteristics of the battery were checked, the capacity retention rate after 100 cycles was 65.0%.

Table 1 shows the capacity retention rates of the batteries obtained in Examples 1-3 and Comparative Examples 1 and 2. The batteries obtained in Examples 1-3 exhibited good charge-discharge cycle characteristics.

As can be seen from Table 1, the charge-discharge cycle characteristics of lithium-ion secondary batteries can be improved by using an electrode-forming material containing polyamide-imide and an electrode containing an active material containing both silicon-based and carbon-based active materials.

Claims (7)

It contains polyamide-imide (excluding silane-modified polyamide-imide having an alkoxysilyl group represented by the following general formula (I)),
The number-average molecular weight of the polyamide-imide is 3,000 or more and 30,000 or less (excluding 30,000).
An electrode mixture containing an active material comprising a silicon-based active material and a carbon-based active material, and having a copper -containing current collector (excluding a negative electrode characterized by comprising a Ni-containing current collector foil and a negative electrode active material layer formed on the surface of the Ni-containing current collector foil, comprising a Si-containing negative electrode active material and a polyamide-imide), is used for forming an electrode mixture.
The electrode mixture contains 80% by mass or more of the active material, based on the total mass of components other than the solvent contained in the electrode mixture.
An electrode forming material wherein the content of the silicon-based active material is 15% by mass or more, based on the mass of the active material .
The electrode-forming material according to claim 1, wherein the polyamide-imide comprises a structure derived from a compound represented by the following formula (III).
(In the formula, ^R2 is a hydrogen atom, an alkyl group, a hydroxyl group, or an alkoxy group, and ^R3 is an isocyanate group or an amino group.) The electrode forming material according to claim 1 or 2, wherein the content of the silicon-based active material is 20 to 50% by mass, based on the mass of the active material. An electrode mixture comprising an electrode forming material according to any one of claims 1 to 3, an active material including a silicon-based active material and a carbon-based active material, and a solvent,
The content of the active material is 80% by mass or more, based on the total mass of components other than the solvent contained in the electrode mixture.
The content of the silicon-based active material is 15% by mass or more, based on the mass of the active material.
An electrode mixture used to form an electrode having a copper current collector, which contains an active material comprising a silicon-based active material and a carbon-based active material (excluding a negative electrode characterized by comprising a Ni-containing current collector foil and a negative electrode active material layer formed on the surface of the Ni-containing current collector foil, which comprises a Si-containing negative electrode active material and a polyamide-imide) . The electrode mixture according to claim 4, wherein the content of the silicon-based active material is 20 to 50% by mass, based on the mass of the active material. An electrode for an energy device, comprising a current collector and an electrode mixture layer formed on at least a portion of the surface of the current collector using the electrode mixture described in claim 4 or 5 (excluding a negative electrode characterized by comprising a Ni-containing current collector foil and a negative electrode active material layer formed on the surface of the Ni-containing current collector foil and containing a Si-containing negative electrode active material and a polyamide imide) . An energy device having a positive electrode and a negative electrode, wherein at least one of the positive electrode and the negative electrode includes an electrode for an energy device as described in claim 6.